Carbon cycling in an old growth forest

Currently ocean and terrestrial biosphere are significant sinks for atmospheric carbon dioxide. The partitioning of the sink into these two reservoirs can be constrained by observations of the mass and ¹³C isotope balance of CO₂ of the atmosphere, as uptake by the terrestrial biosphere discriminates more against ¹³C than does ocean uptake. A large uncertainty in the equations used for the 13C mass balance of the atmosphere is the isotopic disequilibrium that results from secular changes in the δ ¹³C of atmospheric CO₂ that have occurred since the beginning of the industrial revolution. The atmosphere has become more enriched in ¹²CO₂. Because there is a time delay of several years in the cycling of fixed carbon through terrestrial ecosystems, the isotopic composition of CO₂ released by respiration from the ecosystem can be more enriched in ¹³C than that which is currently being fixed. This disequilibrium of the land carbon fluxes accounts for about 20% of the total isotope balance of atmospheric CO₂, and this term has been difficult to measure. We have constructed an integrated land surface and carbon cycle model that explicitly treats the dynamics of carbon movement through (and the isotopic composition of) multiple carbon pools. We have used this model to simulate carbon cycling, the isotope ratio of major carbon pools and the isotopic disequilibrium in this ecosystem as forced by measured changes in the concentration and isotopic composition of CO₂ over the past 400 years. We compare these simulations to observations of the present carbon isotopic composition of carbon in various pools in the ecosystem, to independent estimates of the turnover times of these pools, and to rates of net CO₂ exchange by the ecosystem. We discuss the constraints that these observations provide on the isotopic disequilibrium flux from this ecosystem and the possible use of this model for estimating the global terrestrial ¹³C-disequilibrium.